Helmets in general are made up of an interior shell and an exterior shell fastened together. In the conventional methods of helmet construction, the exterior shell is usually formed first, and high-temperature-resistant printing ink is coated on the inside surface of the exterior shell. The exterior shell is then placed inside a hot-press mould capable of injecting plastic particles. Under high temperature and pressure, injected plastic particles thaw, froth and expand into firm and light interior shell, which tightly presses against the exterior shell. Since this type of conventional helmet construction process requires high heat and pressure, materials for the exterior shells and coating ink must be high-heat-resistant, high-pressure-resistant and distortion-resistant. Generally, polycarbonate (PC) and high-grade plastic are used as raw materials for exterior shells. High-temperature-resistant printing ink is used as adhesive for fastening the interior and exterior shells. Thus, these raw materials are expensive and the coefficient of adhesion between interior and exterior shells is unsatisfactory. Other shortcomings in the conventional method include complex operating techniques, requirement for high precision of mould matching, short usage life, long production cycle and expensive manufacturing cost.
It is therefore desirable to construct helmets without the foregoing disadvantages.